"If you add another piece to the length of the flute it would probably change the key from C to B? as well as giving you the lower note. I would suspect that the extension would transpose the flute from C to Bb" Kenny.

I dont think the longer foot actually does do that. It just adds another note.

From www.fluteworld.com

B Foot This foot joint reaches down to low B, one half-step lower than the C foot, by adding one more key and extending the length of the flute. The B foot joint adds weight to the overall instrument which increases resistance and results in producing an overall darker tone versus the brighter tone of a C foot joint. The B foot joint improves the overall pitch of the third octave. It is the standard flute choice for contemporary compositions requiring extended techniques.

But I agree that harmonising by palying the 3rd above sounds like a good idea.

Allie